I came up with a new Theory today.

Basically, I think the Spirit King is actually Ichigo's father.

I think Aizen is gonna to through all the trouble of getting to the King's Domain, defeating Zero Squad, and then he's gonna burst into the Spirit King's throne room and it's gonna be empty. Then someone will tell him, "The Spirit King? He fell in love with some human woman like 20 years ago and went to live on Earth."

Which would explain why Ichigo is ridiculously powerful for no real reason. Because he's the son of the Spirit King.